Starting this morning, every time I try to add an item to my iTunes library, a blank window appears. I can close it, and in the main iTunes window there's an exclamation mark in the top right corner. If I click on it, a dropdown appears with the information: Adding item to library cancelled.
For clarity: I am not trying to add new files, I'm just browsing the iTunes catalog and trying to add items from there.
I have an active Apple Music Account so I see no reason why I should not be allowed to add an item from the iTunes Store.
Any suggestions on what the problem might be and how to solve it? Thanks.
MacOS Mojave 10.14.4 / iTunes 188.8.131.52